According to the prior art, a topographical map of a surface can be measured using a camera technique, for instance, using a stereo method, with the aid of structural light, and using various phase-shift techniques. In these methods, several images are typically taken of the same surface, in such a way that the manner of illumination the surface is different in each image. When the manner of illumination is selected in a specific way, these images taken in different ways can be used as a basis to calculate the topography of the surface, as well as to compensate for the effect of some other factors, for example, the texture or colour of the surface, on the result of the calculation. In terms of the methods, it is essential that the same points on the surface can be identified from the images taken of the surface under different kinds of illumination. This is simplest when the surface does not move, so that images illuminated in different ways can be taken temporally consecutively. Similarly, the scattering properties of the surface, for example, can be measured by taking several images of the surface, in each of which images the lighting geometry is different, and by comparing these images with each other.
When the surface is moving, it is more difficult to exploit these methods. The same point on the surface cannot be imaged twice in exactly the same geometry with the measuring geometry remaining constant. One solution is to take several images of the surface simultaneously using different colours. This technique is disclosed in, for instance, EP patent EP97114590: Method and apparatus for automatic inspection of moving surfaces. The problem with colour imaging is that the colours of the surface can distort topographic and scattering property measurements based on colour measurement.
If it is wished to make the apparatus as reliable as possible, mechanical components, such as camera shutters etc., should be avoided.